A wart is a common, non-cancerous skin growth caused by certain strains of the human papillomavirus (HPV). These growths can appear on various parts of the body, including the hands, feet, and face. Warts develop when the HPV virus infects skin cells, leading to an overproduction of keratin, a tough protein that forms the outer layer of skin. While generally harmless, warts can sometimes be bothersome, painful, or cause cosmetic concerns.
Macroscopic Characteristics
Warts often have a rough, grainy, or bumpy texture. They can be dome-shaped, flat, or even thread-like, depending on the type and location on the body. Common warts, frequently found on hands and fingers, typically appear as firm, rough bumps. Plantar warts, located on the soles of the feet, might be flat or grow inward due to pressure from walking, often resembling calluses.
Small, dark specks, often called “black dots” or “seeds,” are a distinguishing feature on many warts. These are not seeds, but clotted capillaries visible due to the wart’s abnormal growth. Paring down the wart’s surface makes these thrombosed capillaries more apparent, and the absence of normal skin lines across the lesion can also be noted.
Microscopic Anatomy
Microscopic examination of a wart cross-section reveals several distinct histological features. One prominent finding is marked epidermal thickening (acanthosis), an abnormal increase in the stratum spinosum’s thickness due to increased keratinocytes. The epidermis also exhibits papillomatosis, an inward growth pattern where finger-like projections extend into the dermis, often with a fibrovascular core containing blood supply.
Within the upper epidermis, HPV-infected cells called koilocytes are present. These squamous epithelial cells show structural changes, including nuclear enlargement, an irregular nuclear membrane, and a clear “halo” (perinuclear vacuolization) around the nucleus. Koilocytes are a hallmark of HPV infection.
The macroscopic “black dots” are microscopically confirmed as thrombosed capillaries within the dermal papillae, which are superficial projections of the dermis. Additionally, hyperkeratosis (abnormal thickening of the stratum corneum) and sometimes parakeratosis (where keratinized cells retain their nuclei) are often present. Elongated rete ridges, which are epithelial extensions projecting into the dermis, may also be observed, often pointing radially towards the center of the wart.
The Viral Connection
The microscopic features of a wart cross-section are a direct consequence of Human Papillomavirus (HPV) infection. HPV, a non-enveloped DNA virus, primarily infects basal keratinocytes (cells in the deepest epidermis), typically entering through small skin breaks. Once inside, the virus replicates and manipulates cellular processes. This leads to uncontrolled keratinocyte proliferation, causing the epidermal thickening seen in warts.
Viral proteins, particularly E6 and E7, interfere with host cell tumor suppressor genes (like p53 and RB) that regulate cell growth and division. This interference promotes the uncontrolled multiplication of infected cells. Another viral protein, E4, contributes to koilocyte appearance by disrupting the squamous cell cytoskeleton, leading to the perinuclear halo and nuclear changes. Continuous viral replication and maturation within differentiating epidermal cells lead to viral particle accumulation, which are then shed with exfoliated skin. This constant cycle of infection and altered cell growth gives warts their persistent nature.
Implications of Internal Structure
Understanding a wart’s internal structure is valuable for accurate diagnosis and effective treatment strategies. Specific microscopic features, such as marked epidermal thickening, papillomatosis, and koilocytes, help differentiate warts from other similar skin conditions. For instance, calluses and corns, also areas of thickened skin, lack the viral-induced cellular changes and thrombosed capillaries seen in warts. A clinical examination, often involving paring down the lesion to reveal the “black dots” of thrombosed capillaries, can aid in this distinction.
The internal anatomy of warts also guides various treatment approaches. Topical treatments, such as salicylic acid, work by slowly destroying the virus-infected epidermal cells and softening the thickened keratin layers. Cryotherapy, which involves freezing the wart with liquid nitrogen, targets the infected cells and the blood supply within the dermal papillae, leading to the formation of a blister and subsequent shedding of the wart. Laser treatment also focuses on destroying the tiny blood vessels that supply the wart, effectively cutting off its nourishment. Surgical removal, while an option, can be associated with scarring and recurrence, emphasizing the importance of understanding the wart’s depth and spread.